Screening mammography has been the gold standard for breast cancer detection for over 30 years. However, the sensitivity of screening mammography varies considerably. The most important factor in the failure of mammography to detect breast cancer is radiographic breast density. In studies examining the sensitivity of mammography as a function of breast density, it has been determined that the sensitivity of mammography falls from 87-97 percent in women with fatty breasts to 48-63 percent in women with extremely dense breasts.
Diagnostic alternatives to mammography include ultrasound and MRI. The effectiveness of whole-breast ultrasound as a screening technique does not appear to be significantly different from mammography. MRI has a high sensitivity for the detection for breast cancer and is not affected by breast density. However, since bilateral breast MRI is currently approximately 20 times more expensive than mammography, it is not in widespread use as a screening technique.
Another prior-art technology is positron emission mammography (PEM). This uses two, small, opposing PET detectors to image the breast. The PEM technology offers excellent resolution; however, the currently available radiotracer (F-18 Fluoro deoxyglucose) requires that a patient fast overnight, the patient must have low blood levels (this is often a problem for diabetics), and after injection, the patient must wait 1-2 hours for optimum uptake of F-18FDG in the tumor. The high cost of these PET procedures coupled with the long patient preparation time reduces the usefulness of this procedure and makes it difficult to employ for routine breast evaluation.
Radionuclide imaging of the breast (scintimammography) with Tc-99m sestamibi was developed in the 1990s and has been the subject of considerable investigation over the last 10-15 years. This functional method is not dependent upon breast density. Large multi-center studies have shown the sensitivity and specificity of scintimammography in the detection of malignant breast tumors to be approximately 85 percent. However, these results only hold for large tumors and several studies have shown that the sensitivity falls significantly with tumor size. The reported sensitivity for lesions less than 10-15 mm in size was approximately 50 percent. This limitation is particularly important in light of the finding that up to a third of breast cancers detected by screening mammography are smaller than 10 mm. Prognosis depends on early detection of the primary tumor. Spread of a cancer beyond the primary site occurs in approximately 20-30 percent of tumors 15 mm or less in size. However, as tumor size grows beyond 15 mm, there is an increasing incidence of node positive disease, with approximately 40 percent of patients having positive nodes for breast tumors 2 cm in diameter. Hence, for a nuclear medicine technique to be of value in the primary diagnosis of breast cancer, it must be able to reliably detect tumors that are less than 15 mm in diameter. The failure of conventional scintimammography to meet this limit led to its abandonment as a useful technique in the United States.
In an attempt to overcome the limitation of conventional scintimammography, several small field-of-view gamma cameras have been developed that permit the breast to be imaging in a similar manner and orientation to conventional mammography. One commercial system for single photon imaging that is currently available is that manufactured by Dilon Technologies of Newport News, Va. Using a small detector and compression paddle, they reported a sensitivity of 67 percent for the detection of sub-10 mm lesions.
These systems employ a small gamma-ray camera that is attached to a mammography unit or to a stand-alone system in such a way that the gamma-ray camera is proximate to or in direct contact with a breast compression system. The system includes two identical opposing CZT detectors and performs planar imaging of the breast under compression. Recent clinical studies with the dual-head system have shown an increase in sensitivity to nearly 90 percent for lesions less than 10 mm.
Despite this improved percentage of success, the failure to identify lesions of any size can have significant consequences. Accordingly, it would be desirable to have a system and method to provide additional information to aid in the process of diagnosis, analysis, and treatment planning.
To overcome the aforementioned deficiencies, a molecular breast imaging (“MBI”) system was introduced by providing a system for performing quantitative tumor analysis using information acquired with a dual-headed molecular breast imaging system. Specifically, the MBI system utilize the information available in planar dedicated breast imaging to provide previously unavailable information sets to aid in the diagnosis and biopsy of the site. MBI system can accurately determine the size, depth to the collimator, and relative tracer uptake of a tumor. The MBI system employs two small opposing gamma camera detectors. The breast is compressed between the two gamma cameras and radiation emitted by single-photon radiopharmaceuticals, such as Tc-99m sestamibi, is detected by collimation.
Although multiple possible radiotracers are available for breast imaging with MBI, Tc-99m Sestamibi is FDA approved and is the most widely used tracer. Because MBI involves intravenous injections of the radiotracer, it poses a very different type of radiation risk than x-ray mammography. The radiotracer distributes throughout the body, exposing many organs and tissues to radiation, in contrast to mammography, in which the only organ affected by radiation is the breast. So, even though the radiation dose to breast tissue is low with MBI, the dose to other organs, and the consequential radiation risk, can be higher than desired. Current techniques employ an administered dose of 20-30 mCi Tc-99m sestamibi that results in an effective dose to the body of 6.5-10 millisievert (mSv). This is about 2-3 times the annual exposure from natural background radiation in the U.S. (˜3 mSv) and is ˜7-10 times that of mammography. Thus, there is a need to substantially reduce the radiation dose from MBI (by a factor of 5-10), so that this technology can compete against mammography in a screening environment. However simply reducing the administered dose of Tc-99m sestamibi is not possible as the increased noise level in the resulting images significantly degrades image quality. Hence, both hardware and software enhancements need to be made to the technology in order to achieve this level of dose reduction while maintaining image quality.
With that said, the MBI system has been shown to have a relatively high sensitivity (>90%) for the detection of sub-10 mm lesions. In addition, studies have shown that MBI detected 3 times as many cancers as digital and analog mammography in asymptomatic women at increased risk of breast cancer. More recent studies have found the sensitivity of MBI to be comparable to that of MRI. Hence, MBI appears to be a very attractive alternative to mammography, particularly in women at increased risk of breast cancer and in women with dense breast tissue on this mammogram.
Accordingly, it would be desirable to have a method that can obtain satisfactory images with MBI for both the diagnosis and screening of breast cancer by using a reduced radiotracer dose.